I think it sounds to me like you have social anxiety. I am like that too. I get over-stimulated when I'm around big groups of people. Like, when I used to be social and would go out with Meetup groups, I'd get so over-stimulated I'd be up all night unable to fall asleep or function the next day. I don't think social anxiety is equal to or the same as having a mental illness. But I know people may disagree with that belief I have.